Currently, conventional die stacking processes are incapable of successfully bonding a die to a wafer with the desired precision for some applications. For example, there are applications that call for bonding a die containing one type of circuitry (e.g., digital processing circuitry) to a wafer containing another type of circuitry (e.g., analog circuits and memory). The die includes electrical connectors (e.g., exposed conductors or pads) that contact and connect with reciprocal connectors on the wafer. For successful bonding, the connectors need to be aligned to each other before bonding, so that reliable electrical connections are formed when the die is bonded to the wafer. However, as device geometries continue to shrink, it becomes more difficult to align the die to the wafer (and more specifically the connectors of each) before bonding so that the electrical connections between the two are made at the point bonding occurs. Obtaining the desired alignment can require very expensive and complex alignment equipment. Moreover, pressing connectors together does not always generate an immediate and/or long lasting electrical connection.
One solution has been proposed where alignment structures are formed adjacent the bond site to guide a misaligned die into proper alignment during bonding. As the die is lowered onto the wafer, if there is any misalignment, the die physically hits the alignment structure and is moved laterally by that physical contact such that by the time the die reaches the wafer, the two are properly aligned to each other. Conventional attempts using this alignment technique have used materials such as Al, silicon dioxide, or silicon nitride for the alignment structure. However, these materials lack sufficient elasticity to effectively guide the die laterally upon physical contact (there is excessive damage to both the alignment structure and the die), and it was difficult to create deep enough alignment structures using such materials. The collision of the die with such rigid alignment structures does not effectively guide the die into proper position. Chinese patent publication CN 102403308 proposed using a polymer for the alignment structure, but it did not identify any specific polymer to implement this solution. While many types of polymers are more elastic than Al, oxide or nitride, they are too soft at the high temperatures necessary during bonding (e.g., greater than 100 C) to act as alignment structures, and they typically burn at such temperatures.
There is a need for an alignment structure and technique that reliably aligns die to wafer without using expensive and complex alignment equipment, yet effectively allows for the creation of electrical connections between die and wafer when bonded together.